Antibiotics can kill bacteria by interfering with their normal intracellular functions (RNA, DNA and protein synthesis) or by weakening the bacteria's cellular structure, causing the cell to break open or lyse. Unlike animal cells, bacterial cells have both a plasma membrane and a stiff outer cell wall. Antibiotics that disrupt either the cell wall or plasma membrane are described.
Penicillin and its Derivitives
Penicillin functions by blocking a specific cross-linking step in cell wall production that is critical for the bacteria to form strong cell walls. Penicillin blocks the action of the cross-linking enzyme transpeptidase, so although cell wall subunits are produced normally, they are not cross linked to one another. This cross-linking failure creates weak bacterial cell walls that cannot contain the bacterial cytoplasm and the cell lyses, killing the bacteria. Penicillin, amoxicillin and ampicillin are examples of antibiotics that work by blocking the cross-linking step.
Vancomycin also acts to weaken newly made bacterial cell walls by interfering with the synthesis of the cell wall subunits (peptidoglycans). Whereas penicillin and its derivatives act directly on the cross-linking enzyme to block its enzyme action, vancomysin acts by attaching itself to the precursors of the subunits, blocking the substrate from interacting with the enzyme. Without proper cross-linking of subunits, the bacterial cell wall becomes weak and lyses. Vancomycin's other modes of action are to increase the permeability of the cell membrane and to inhibit bacterial RNA synthesis, making it a very powerful triple-threat antibiotic.
Bacitracin blocks yet another step in bacterial cell wall synthesis. The subunits of the cell wall (peptidoglycans) are produced inside the cell and must be transported through the lipid-rich cellular membrane. A special transporter molecule called a lipid carrier molecule carries the peptidoglycan through the membrane, releases the subunit and is recycled back through the membrane to pick up the next subunit. Bacitracin blocks the recycling of the lipid carrier molecule needed to transport the next subunit, weakening the cell wall. The weakened cell wall lyses, killing the bacterial cell.
Polymyxin B, a cyclic peptide antibiotic, acts on another component of cell structure, the plasma membrane. Polymyxin acts by increasing the permeability of the cell membrane, causing the cell to take up too much water and lyse, killing the bacterium. Unfortunately, polymyxin also interacts with the cell membrane of human kidney and nerve cells, making it toxic for these organs and therefore a relatively poor internal antibiotic for clinical use. Polymyxin is commonly used in topical antibiotic preparations.